This invention relates in general to a coating system and, more specifically, to a system for immersion coating of drums.
Electrostatographic imaging members are well known. Typical electrophotographic imaging members include photosensitive members (photoreceptors) that are commonly utilized in electrophotographic (xerographic) processes in either a flexible belt or a rigid drum configuration. These electrophotographic imaging members comprise a photoconductive layer comprising a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous charge transport layer and the supporting conductive layer. Alternatively, the charge transport layer may be sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a member such as paper.
Electrostatographic imaging drums may be coated by many different techniques such as spraying coating or immersion (dip) coating. Dip coating is a coating method typically involving dipping a substrate in a coating solution and taking up the substrate. In dip coating, the coating thickness depends on the concentration of the coating material and the take-up speed, i.e., the speed of the substrate being lifted from the surface of the coating solution. It is known that the coating thickness generally increases with the coating material concentration and with the take-up speed.
The need for faster printing speed, e.g., up to about 75 pages per min and printing two pages side by side has lead to the development of long, large diameter drum substrates, instead of shorter small diameter drums. The larger diameter drums also provide more surface area around the periphery of the drum to locate large, space consuming development stations. For such requirements, the drum dimension can be 38 centimeters (15 inches) in diameter and 76 centimeters (30 inches) in length. As the size and weight of the substrate is increased, the problems involving in inserting and withdrawing the substrate from a coating vessel are compounded. Thus, for example, dip coating of large heavy hollow cylinders requires large quantities of coating liquid which can be wasteful if the coating liquid has a short pot life. Moreover, vibration or wobble during transport of a large heavy drum into and out of a coating liquid can cause undesirable coating defects.
Another technique for immersion coating comprises (a) positioning the substrate within a coating vessel to define a space between the vessel and the substrate and providing a downwardly inclined surface contiguous to the outer surface at the end region of the substrate; (b) filling at least a portion of the space with a coating solution; and (c) withdrawing the coating solution from the space, thereby depositing a layer of the coating solution on the substrate. This process is described in U.S. Pat No. 5,616,365, the entire disclosure thereof being incorporated herein by reference. When this process is utilized for coating a large drum, e.g. 24 centimeters (9.5 inches) in diameter, in which coating fluid is withdrawn at the bottom to deposit a coating layer on the drum located in the center of a coating vessel, it can produce uniform and defect free coating for thin undercoating layers and thick charge transport layers. However, when drums are dip coated in essentially a closed environment using a minimum amount of coating fluid and without requiring precise motion control of potentially massive substrates over large distances as in the system described in U.S. Pat. No. 5,616,365, it has been found that substrate is not always centered when inserted into a coating vessel for a coating operation. Non-uniform coatings can be formed when the spacing between the outside surface of the drum being coated and the adjacent coating vessel wall is not uniform completely around the outer surface of the drum.
These defects are unacceptable for high printing quality requirements such as extremely uniform thickness and defect free coatings. Solutions to these coating problems are crucial for complex, advanced precision tolerance imaging systems.